Thermoplastic reins, particularly polycarbonate resins, are widely utilized as engineering plastics in various fields such as in the fields of OA appliances and electric and electronic appliances and in the field of automobiles, because of their excellent mechanical strengths (particularly impact resistance properties), electric properties and transparency. In these utilization fields, there are some fields that require flame retardancy. The center of such utilization fields is the field of OA appliances and electric and electronic appliances. Among various thermoplastic resins a polycarbonate resin is one of the resins having a high oxygen index and is generally said to be a self-extinguishable resin. The level of flame retardancy required in the field of OA appliances and electric and electronic appliances is, however, generally as high as a V-0 level in terms of UL94 standard pertaining to flame retardancy. Thus, in order to impart a flame retardancy that meets with such a level to the composition, it is a general practice to add a flame retardant or a flame retarding aid thereto.
On the other hand, a polycarbonate-polyorganosiloxane copolymer or a mixture of a polycarbonate-polyorganosiloxane copolymer with a polycarbonate resin is generally known to exhibit a higher flame retardancy as compared with a polycarbonate resin. Since the flame retardancy attained by using a polycarbonate-polyorganosiloxane copolymer by itself is still insufficient to satisfy the above flame retardancy level, compositions in which various flame retardants are cojointly used are disclosed (for example, Japanese Patent Application Laid-Open No. Sho 63-S-63-289059, Japanese Patent Application Laid-Open No. Hei 1-210462, Japanese Patent Application Laid-Open No. Hei 3-200862 and Japanese Patent Application Laid-Open No. Hei 4-202465). In the techniques disclosed in these publications, however, there is a drawback that a melt of the polycarbonate-based resin drops when the resin is highly fluidized during combustion. Also, a system in which a flame retardant, particularly a halogen-containing flame retardant such as a bromine compound, generally poses a problem that the thermal stability is poor.
As described previously, polycarbonate resins are widely used as OA appliances, electric and electronic appliances and automobile parts, because of their excellent mechanical strengths (particularly impact resistance properties), electric properties and transparency. It is necessary for the materials for such precision parts to give moldings having good surface smoothness and dimensional stability. Further, the moldings obtained are required to have a good appearance.
In this circumstance, the present inventors have proposed a resin composition which comprises a polycarbonate-polyorganosiloxane copolymer blended with a polytetrafluoroethylene and which has improved thermal stability and melt dropping property and yet shows high flame retardancy (Japanese Patent No. 3037588). However, still further improvement of the flame retardancy is desired.
Japanese Patent Application Laid-Open No. Hei 5-214234 discloses a resin composition including a polycarbonate-polyorganosiloxane copolymer blended with a polytetrafluoroethylene and fumed silica. Fumed silica, which has a network structure, however, is not able to be sufficiently dispersed in the resin. Therefore, in order to obtain a high impact strength, it is necessary to increase the content of the siloxane. As a consequence, it is not possible to obtain moldings having a satisfactory appearance. Japanese Patent Application Laid-Open No. Hei 6-172628, Japanese Patent Application Laid-Open No. Hei Japanese Patent Application Laid-Open No. Hei 7-33971 and Japanese Patent Application Laid-Open No. Hei 9-310011 disclose techniques in which silica is added to various resins including a polycarbonate. In these techniques, granular or powdery silica having specific shapes are added for the purpose of improving mold processability, drop-preventing property during combustion and blow-moldability as well as flame retardancy. With such methods, the silica is not able to be sufficiently dispersed in the resin and, therefore, it is impossible to produce moldings having satisfactory flame retardancy, impact strength and appearance.